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Mooring systems for marine energy converters
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Flory, JF; Banfield, SJ; Ridge, IML; et al.
DEPOSITED IN ORE
20 January 2017
This version available at
http://hdl.handle.net/10871/25315
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Mooring Systems for Marine Energy Converters
John F. Flory, Tension Technology International, Morristown, NJ, USA, [email protected]
Stephen J. Banfield, Tension Technology International, Eastbourne, UK, [email protected]
Dr. Isabel M.L. Ridge, TTI Testing, Wallingford, Oxfordshire, UK, [email protected]
Ben Yeats, Tension Technology International, Inverness, UK, [email protected]
Tom Mackay, Tension Technology International, Inverness, UK, [email protected]
Dr. Pengzhu Wang, Bridon International, Doncaster, UK, [email protected]
Tim Hunter, Bridon International, Doncaster, UK, [email protected]
Prof. Lars Johanning, Uni. of Exeter, Exeter, UK, [email protected]
Manuel Herduin, Uni. of Western Australia, Crawley, Australia, [email protected]
Peter Foxton, Vryhof Engineering Limited, Douglas, Isle of Man, UK, Peter. [email protected]
Abstract
This paper discusses several new technologies for mooring floating marine energy converter (MEC) devices, such as wave energy generators, tidal current turbines and floating wind turbines.
The principal mooring component is a special nylon fiber rope which provides cyclic tension fatigue endurance much superior to that of conventional nylon ropes. The nylon fiber is treated with a new proprietary coating which has excellent wet yarn abrasion properties. The parallel‐subrope type rope construction further reduces internal abrasion.
Extensive laboratory testing was carried out on this new nylon rope design. Cyclic tension fatigue tests were conducted at mean loads and load amplitudes typical of actual service conditions and at higher mean loads and amplitudes. These tests demonstrate that the special nylon rope has essentially the same, desirable stretch characteristics as conventional nylon rope and has much better endurance performance.
The mooring connection to the floating MEC device consists of a high‐modulus fiber rope pendant which passes through a low‐friction bell‐mouth nylon fairlead on the MEC device. This eliminates the use of heavy, unreliable chain in this critical connection.
A unique bag anchor system would be used on sand, clay, rock and other sea beds in which conventional drag embedment anchors and driven piles are impractical. The bag anchor consists of a large abrasion resistant carcass with lifting straps and top closure. The bag is transported to site in a collapsed form and is filled with local sand or aggregate to provide ballast weight. Several or many such bags are enclosed within a fiber rope net for deployment and are grouped together for connection to the mooring line.
The paper will be of particular interest to designers of moorings for MEC systems in shallow water and severe wave environments. It will also be of interest for other mooring applications.
To be presented at OCEANS ’16 Monterey MTS/IEEE Conference, Sept. 19‐ 22, 2016 Copyright IEEE
Key Words – energy converter, wave energy, current energy, wind energy, buoy, mooring, anchor, rope, nylon, fairlead, test
INTRODUCTION
Marine energy converters (MEC) will generally be located in very demanding environments, often in shallow water depths and often on sand, clay, rocky or hard sea beds. The mooring systems for these MECs should keep the device on station in severe wave, current and wind conditions. They should also limit horizontal offset in order to accommodate power cables to the ocean floor. The mooring system footprint should be small to control installation costs and to reduce the area occupied by arrays of devices.
Conventional steel wire and chain mooring systems do not provide the necessary compliance (stretch) and reliability. A mooring component failure will necessitate high offshore repair costs or the need to take the device out of service.
With funding from the UK Carbon Trust and other agencies, Tension Technology International (TTI), Bridon International, Exeter University, Vryhof and other companies and institutions are developing and testing components for a MEC mooring system which provides the necessary strength, compliance and reliability.
MEC systems are typically moored in less than 100 m (300 ft) of water. The maximum peak to trough wave might be as high as 25 m (75 ft), with corresponding large wave‐induced horizontal excursions. Thus in survival mode, the MEC mooring system might have to accommodate horizontal movements which are more than 25% of the water depth.
A schematic of a catenary anchor leg moored buoy is shown in Figure 1. Catenary mooring systems rely on the weight of chain to produce restoring forces. As the buoy is offset to the side by waves or other forces, the chain becomes taut and pulls up on the anchor. The mooring leg becomes stiff and cannot offset any further, resulting in very high tension.
A taut‐leg fiber anchor leg moored buoy is also shown in Figure 1. This type of mooring is now used on many deepwater oil exploration and production platforms. In such applications, the fiber rope mooring legs typically extend down at about 45 degrees to anchors on the sea floor. As the buoy is offset, the mooring rope stretches, but it does not become as stiff as a fully offset catenary anchor chain.
The taut‐leg fiber rope anchor leg mooring system now proposed for use on MECs is shown in Figure 2. A nylon rope mooring line is preferred because it has much more stretch than polyester rope. A high modulus fiber rope pendant is used to connect the mooring line to the buoy, eliminating the use of chain. The pendant passes through a low friction fairlead on the buoy. A bag anchor system would be used where conventional drag or pile anchors are not practical.
Nylon Mooring Rope
The principal mooring component is a special nylon fiber rope which provides the compliance (stretch) and
cyclic tension fatigue performance necessary to moor MEC devices. The fatigue durability is much superior
to that of conventional nylon ropes. The unit length cost of nylon is approximately half that of chain with
the same break strength. In shallow water moorings, larger chain would normally be used to provide the
necessary catenary mooring offset characteristics. Thus using nylon mooring lines can achieve substantial
cost benefits.
Research in the 1980s showed that, when subjected to cyclic tension, wet conventional nylon ropes degrade
rapidly due to internal abrasion.1 2 Special treatments to the nylon yarn were shown to greatly improve
cyclic tension fatigue performance.3 4
Because polyester ropes do not suffer from this internal abrasion problem, they are recommended for many
mooring applications.5 Research demonstrated that polyester mooring ropes have excellent durability when
subjected to cyclic tension.6 7 8
Polyester ropes are now widely used for mooring oil exploration and production platforms in deep water.9
One objective in this application is to minimize platform horizontal excursion in order to prevent
overstressing the drill stem or production riser. Polyester rope taut leg mooring systems in water depths
greater than 1,000 m (3,000 ft) can limit platform excursions to less than 2% of water depth.
MEC devices are typically moored in less than 100 m (300 ft) of water. Polyester ropes could be used in this
application, but very long mooring legs would be needed to provide the necessary stretch. Shorter nylon
ropes would provide the necessary stretch. But conventional nylon ropes would have very limited service
life in this application. If nylon rope mooring legs could be used, it would save cost and reduce the footprint
of the mooring arrangement.
Special Parallel Subrope Type Nylon Rope
Bridon has designed a special nylon rope which has much better endurance in tension cycling. Some test
results for small rope specimens were discussed in a previous paper.10 This paper provides additional test
results and further information.11
Most conventional nylon mooring ropes are either double‐braid or 8‐strand construction. Figure 3 shows a
double braid rope. It comprises a braided core surrounded by a braided cover. Internal abrasion takes place
at the many cross‐over points between the braided strands.
Figure 3 also shows the parallel‐subrope type rope construction. It comprises many subropes which extend
in parallel over the entire rope length and are covered by a braided jacket. The large polyester ropes used
to moor deepwater platforms are of this construction. There is essentially no abrasion between the parallel
subropes. These subropes are typically long‐lay‐length 3‐strand construction which further reduces internal
abrasion.
The nylon yarn used in this Bridon subrope is treated with a new proprietary coating which has excellent
wet yarn abrasion properties. Figure 4 compares the wet yarn abrasion performance of this nylon yarn with
that of another nylon yarn and with the Cordage Institute criteria for marine grade nylon yarn.12
In the yarn‐on‐yarn abrasion test, a yarn under tension is rubbed against itself until it fails.13 The Cordage
Institute minimum performance criteria for marine grade nylon yarn are shown on this graph as a dashed
line.14 Type 1, an earlier nylon yarn tested by Bridon for this application, performed better than most nylon
yarns. The abrasion performance of the Type 3 yarn is more than ten times that of the Cordage Institute
criteria. Bridon now uses the Type 3 nylon yarn in this new rope design.
Extensive laboratory testing has been carried out to demonstrate the endurance of this new nylon rope
design. A first series of tests, reported in the previous paper, were conducted on a small 17 mm (0.7 inch)
diameter subrope. That rope has an average wet new break load (NBL) of 68 kN (15.3 kip). It would be
suitable for making parallel‐subrope type mooring lines for small buoys.
This paper presents some of the results of a second series of tests conducted on a larger subrope, shown in
Figure 5. The diameter of that subrope is 33 mm (1.3 inch) and the average wet NBL is 249 kN (60 kip). This
large subrope would be used to make mooring lines for large renewable energy system moorings. Many
such subropes would be arranged in parallel and covered by a braided jacket to make the mooring line.
The lay length to diameter ratio for both ropes was greater than 6:1. This is a longer lay length than that
used in conventional 3‐strand nylon ropes. The subrope can have such a long lay length because the
assembled mooring rope is jacketed.
Figure 6 compares the broken‐in stiffness (after 10 cycles) of this special nylon subrope with a comparable
polyester subrope used in parallel strand ropes. At 50% of NBL, the strain in the polyester subrope is about
5% of its length, and the strain in the nylon subrope is about 13%. These strain values are similar to those
for conventional polyester and nylon rope constructions.
Cyclic tension fatigue tests were conducted on both the small and large nylon subropes. Figure 7 is a cycles
to failure (CTF) plot of the results of these tests. There is essentially no difference between the fatigue
performance of the recently tested large subrope and that of the small subrope results reported in the
previous paper. A dashed line shows the mean of these CTF data, determined by regression analysis.
The solid line is 2 standard deviations below the mean CTF line. It represents a suggested design CTF line,
using the format employed in section 6.2 of API RP 2SK for fatigue resistance of mooring components.15
Figure 8 compares the cyclic load design line of this nylon subrope with those of other fiber and wire ropes
and chain. It is similar to that of conventional polyester ropes. It is much better than that of conventional
nylon rope constructions. It is also better than those of wire rope and chain in water.
For a typical renewable energy device mooring under consideration, the mean mooring load is 30% of NBL,
and the cyclic load range is 6% NBL. A small subrope was cycled 10,000,000 times at this mean load and
load range in a test which lasted thirteen months. After this cycling test, the rope’s break strength was
94%of its original wet NBL.
A large subrope was cycled 20,000,000 cycles at that mean load and range. After this test, the rope’s break
strength was 108% of its wet NBL. It is not unusual for rope strength to increase during cycling. Examination
of the rope interior after the break showed no apparent signs of abrasion wear.
Nylon Low‐Friction Fairlead To Reduce Wear
A high‐strength fiber rope pendant can be used as the connection to the MEC device instead of heavy,
unreliable chain. The fiber rope pendant would pass through a low‐friction nylon bell‐mouth fairlead and a
plastic lined hawse pipe. This work was funded by Innovate UK under the Marine Energy Supporting Array
Technologies (MESAT) program.
The bell‐mouthed fairlead is based on the nylon Panama fairlead developed by TTI and marketed by
Nylacast. The material is Oilon grade nylon which has special lubricant to reduce friction and abrasion. That
fairlead design is now used successfully on many gas carriers to reduce wear on synthetic fiber mooring
lines.16
This bell‐mouth fairlead is shown in Figure 9. The ratio of curvature of the bell‐mouth to the rope diameter,
D/d is 31:1. For testing, the fairlead was represented by a large Oilon grade nylon sheave having the same
D/d ratio. It was repositioned before each series of tests to expose a new surface to the rope.
Tests were carried out on rope samples of three candidate materials for making the mooring pendants,
Dyneema, Technora and Vectran.17 The ropes were 32 mm (1.25 in.) diameter 12‐strand braided fiber ropes
manufactured by Bridon. The NBL of each of these ropes was about 83 tonnes. The rope and the fairlead
represented the sizes of those which might be employed in field applications on WEC.
The portion of rope which passed through the fairlead was covered by a 4‐layer protective sleeve designed
to prevent the load bearing rope core from sliding within the rope cover. This sleeve also prevents heat
generated by friction due to sliding over the fairlead from reaching the load bearing core.
The tests were conducted on a large cyclic bend and tension load test machine at TTI Testing in Wallingford,
UK. This special test machine is shown in Figure 10. With this machine, one end of the test rope is cyclically
raised and lowered to wrap around the fairlead while the entire rope is also cyclically tensioned to pull back
and forth through the fairlead. Water was sprayed on the rope during testing.
In a first test series, all three candidate ropes were tension cycled 1419 times at a mean load of 40% of NBL
and a load range of 20% NBL, while being cycled 473 times at angles up to 18 degrees. In these tests the
protection sleeve showed no evidence of damage, and there was no measurable loss of nylon material on
the fairlead. The Vectran and Dyneema rope samples did well in these initial tests.
A second test series was conducted on the Vectran rope and on both single and double Dyneema rope
samples using a test matrix derived to represent 6 months of field service. This imposed over a million bend
cycles and corresponding tension cycles. The mean bend angle throughout these tests was 12.5 degrees,
with cyclic ranges of 6, 12 and 18 degrees. The mean load throughout most of the tests was 5% of MBL with
load ranges up to 9% of MBL, but mean loads as high as 40% MBL with a load range of 20% were applied.
In these tests, the single Dyneema rope retained 64% of its new NBL. A second test was conducted with a
doubled Dyneema rope to effectively double the strength and halve the rope stresses. In this test, the
protection sleeve was placed by hand rather than braided by machine, and it was not firmly fixed in place.
It slid along the rope and wore through, allowing the rope to rub directly on the nylon sheave. This caused
the rope to heat up, weakening the HMPE fiber. As a result, that rope failed at 60% of its new NBL. This
problem actually proved that the sleeve effectively serves as a thermal and abrasion resistant barrier to
protect the rope.
After this test with the doubled rope, the internal fibers were in much better condition compared with the
single rope. But bending and rubbing against the fairlead caused thermo‐mechanical working of the fibers
which altered the crystallinity of the HMPE material and caused the highly drawn molecular structure to
revert towards its original undrawn state. The HMPE yarn lost strength as a result. This is a known
characteristic with highly drawn rope making fibers and is especially a problem with highly drawn HMPE.
The 4‐layer protective sleeve combined with the nylon liner performed well in protecting the pendant rope
from external abrasion and frictional heating due to sliding over the fairlead.
These tests demonstrated that a special nylon bell‐mouth fairlead in combination with a special 4‐layer
sleeve can successfully protect a synthetic fiber rope. A larger high modulus rope pendant with a properly
applied protective sleeve would have performed much better. There was no significant damage to the nylon
surface of the sheave which represented the bell‐mouth fairlead.
The tests also identified the HMPE heat‐induced strength loss problem and demonstrated a method of
reducing and eliminating the problem.
Bag Anchors
A unique bag anchor system has been developed for use on sand, clay, rock, mud and other sea beds in
which conventional drag embedment anchors and driven piles are impractical. It is a flexible bag filled with
ballast appropriate for the specific site and application. The ballast is a heavy, pourable material, such as
local sand or aggregate, or heavier “drilling” mud or scrap steel pellets.
The bag anchor functions as a conventional gravity type anchor. A group of anchor bags is enclosed within
a fiber rope containment net and is joined to the mooring line by yokes, as shown in Figure 11.
Each cylindrical bag is about 2 m (6.6 ft) diameter and 2 m high. The bag consists of an abrasion resistant
carcass with lifting straps and top closure. An inner impermeable liner might be used where the ballast
poses an environmental concern. Lifting straps are sewn to the carcass.
The anchor bag is transported to the site in a collapsed form. The bag is filled with ballast weight on shore
or offshore prior to deployment or it is filled after being placed on the seabed.
The bag anchor concept can be economical when compared with alternative gravity anchor systems. A bag
anchor would cost about $250 per wet kip (1000 lb weight in water) using local aggregate and maybe cost
less with denser material. Estimates show that cast reinforced concrete and scrap steel chain would each
cost about $600 per wet kip and that fabricated steel gravity anchors would cost about $1000 per wet kip.
The bag anchors could be delivered to the site empty, thus avoiding shipping costs incurred with the
alternatives.
The bag anchor concept was originally conceived by TTI. It is being developed through a consortium with
anchoring specialists at Vryhof Engineering, the University of Exeter and TenCate. The bag anchor
development, along with the nylon rope development was funded by the UK Carbon Trust under the Carbon
Trust Marine Energy Accelerator Grant and the Scottish Government Marine Renewables Commercilisation
Fund.18
The investigation of feasible mooring systems began in 2008. The bag anchor design was developed in 2009.
Small scale testing of bags, nets and deployment methods were carried out during 2010. A prototype bag
anchor, using geotextile bags, was manufactured in 2011. Lifting trials were carried out on partially filled
bags at the University of Exeter at Falmouth in 2012.
The empty bag anchors were then transported to the Isle of Man in 2013. An assembly of seven anchor
bags was placed on the Douglas harbor floor at low tide, enclosed within a containment net, and filled with
ballast. This arrangement is shown in Figure 12. After the assembly was submerged at high tide, a harbor
tug pulled on it at an upward angle with the tugs full bollard pull capacity of 5 tonnes (11 kip). No
displacement of the assembly was observed. This demonstrated a friction coefficient on the seafloor of at
least 0.45, and it is probably substantially higher.19
A full‐size bag anchor system was manufactured in 2015 for future deployment at a test site. Continuation
of the test program is now awaiting further funding.
Conclusions
The nylon parallel subrope type rope design has been thoroughly tested to demonstrate its suitable for use
on MEC moorings and other similar shallow water buoy and platform moorings. The nylon parallel subrope
type rope has more stretch but less endurance than the similar polyester parallel subrope type rope. It has
better endurance than chain and wire in water and much better endurance than conventional nylon rope
constructions. When more stretch is needed than can be provided by polyester rope, the nylon parallel
subrope design is a very good candidate for use in mooring systems.
The low friction nylon bell‐mouth fairlead has been demonstrated to be suitable for use on buoys and
platforms to protect fiber rope pendants. However, further development and testing of the high modulus
fiber rope mooring pendant should be done. When fully developed, a fiber rope mooring pendant used with
the low friction fairlead can replace heavy, failure‐prone chain on buoys and platforms.
The bag anchor system has undergone limited testing. A prototype bag anchor system is ready for
deployment but is awaiting funding. It can be used as a gravity anchor on rock, sand, clay and other sea
beds on which drag anchors and piles are impractical. The bag anchor system can be more economical than
other gravity anchor material alternatives.
Each of these new mooring system components can be used independently with other conventional
components (e.g. nylon mooring line with drag embedment anchor). The high modulus fiber rope through
low friction fairlead and the bag anchor system concepts require further development. The nylon parallel
subrope type rope is ready for use on MEC device moorings and other suitable mooring applications.
ACKNOWLEDGEMENTS
This work would not have been possible without the funding and support of the Scottish Government, the Carbon Trust and Innovate UK.
The project was funded under the Marine Renewables Commercialization Fund (MRCF) and Marine Energy Supporting Array Technologies (MESAT).
Other partners who contributed to this project include Lloyd’s Register, DNV‐GL, TenCate, Orion Energy Centre, Nylacast, and offshore wind developer IDEOL.
Input and encouragement was provided by tidal power developer partner Bluewater, and wave energy developers AWS Ocean Energy and Pelamis.
July 16, 2016
References
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2 S.H. Markussen, E.S. Hunt and R.E. Hobbs, “Wear of Nylon Hawsers Over Rollers, Pulleys and Fairleads”, Offshore Technology Conference, OTC 4765, SPE, 1984
3 J.F. Flory, J.W.S. Hearle and M. Goksoy, “Abrasion Resistance of Polymeric Fibres In Marine Conditions”, 2nd International Conference on Polymers in a Marine Environment, IME, London, 1987
4 J.F. Flory, Prof. John W. S. Hearle and Dr. Mustafa Goksoy, ‟Yarn Friction and Abrasion Characteristics as Indicators of Rope Performance", MTS '90 Conference Proceedings, MTS, Washington, DC, 1990
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6 NDE/TTI Joint Industry Study, “Engineer’s Design Guide for Deepwater Fibre Moorings”, OPL. London, 1999
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11 I.M.L. Ridge, “Nylon Ropes for WEC Moorings ‐ Final Report”, Carbon Trust Reference Number 0912‐050, TTI, Eastbourne, UK, 2013
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17 S.J. Banfield, Final Report, Synthetic Fiber Rope Polymer Lined Fairleads, TSB Project Ref 19116‐141146, TTI, Eastbourne UK, April 29, 2016
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Figure 1 Catenary Anchor Leg and Taut‐leg Fiber Rope Mooring Systems
Figure 2 Marine Energy Converter Buoy Mooring System, with Bag Anchor System,
Nylon Parallel Subrope Mooring Line and Bell‐Mouth Fairlead
Figure 3 Double Braid and Parallel Subrope Constructions
Figure 4 Bridon Subrope Nylon, Yarn‐on‐Yarn Abrasion Performance
Figure 5 Bridon Nylon 3‐Strand Subrope
Figure 6 Nylon and Polyester Subrope Force vs. Strain Graphs
Figure 7 Nylon Subrope Cyclic Tension Fatigue Test Results
Figure 8 Comparison of Cyclic Tension Fatigue Performance
Figure 9 Nylon Bell‐Mouth Fairlead
Liner for High‐Strength
Fiber Rope Mooring Pendant
Figure 10 Cyclic Tension and Bend Test Machine, TTI Labs, Wallingford
Figure 11 Tandem Anchor Bag System with Net and Connecting Yoke
Figure 12 Prototype Bag Anchor System on Harbor Seafloor, Isle of Man, for Pull Tests
